Or 'energy crops': any energy grass that yields high B.T.U.s (British Thermal Units) per kilogram used as an alternative 'clean' power source.

One good example of e-grass is grassohol: ethanol extracted from switchgrass.
Other plant-source enregy crops are derived from cooking-oil!
'Greasel' (grease diesel), one like what is called 'M-Diesel': methanol mixed with cooking-oil.
Some interesting alternative energy sources are MFCs (microbial-fuel cells: they are mainly, bacteria that turn waste into fuel known as 'living batteries' or 'biobatteries'), sometimes used in gastrobots (see, smart dust) that live off sugar as food!
One last biological innovation are 'biosolar' cells that synthesises protiens to generate electricity!